In seismic exploration, generated acoustic waves are transmitted downward into the earth. At the transition zones between various types of rock strata, reflections of the acoustic waves occur; as attenuated parts of the wave are returned to the earth's surface they are detected by groups of planted geophones. Each geophone group provides a composite signal that is associated with a particular geographic station on the earth's surface. The signal can be the result of summing the outputs of any number of geophones. Usually, for ease of field use, minimum numbers of geophones, say nine, are semi-permanently fastened together to form a basic field group module. It is that module that is called a "flyer" or fly. Members of the field crew transport literally hundreds of these flyers into the field. At each of the stations, a member of the crew can electrically connect two or more of the flyers together to form a single channel of output data, that data being associated with a given geographical location in a patterned alignment with neighboring stations. After the acoustic source has been activated, the composite reflected signal from each of these stations say 48, is fed to electrical circuitry for recording and processing; this is called the basic 48 channel recording technique.
Several types of geophysical data acquisition systems for accomplishing the above are now available; for example, in a conventional system, the signal of each geophone group (station responses) is fed by a pair of conductors (along with signals on neighboring pairs of conductors) to the recording truck. Because the multiplicity of pair conductors within the seismic cable which may stretch for miles with at least 48 pairs not being uncommon, likelihood of breakdown can be rather high; also cable impedance is also rather high; and signal response from individual geophone groups can likewise suffer in ways not easily detected at the recording truck, distortion being an example.
In another more modern system, to overcome the overuse of ultra-long conductor pairs within the seismic cable, a series of remote data acquisition and telemetering circuits (RDATC's) are positioned along the traverse of the geophone spread; these RDATC's gather data from a limited number of geophone flyers, then store, amplify, filter, gain control and digitize the analog data to a compatible digital format; the data bits remain stored until commands release them for transmission (telemetering) back to the central recording truck. That is, since each RDATC is capable of storing the digital information interrogation and transmission to the central recording truck of that data can be in a series format. While RDATC's simplifies the recording of seismic data in a field to a substantial degree, nevertheless, in order to implement electrical connection of the multiplicity of geophone flyers to each RDATC, there may be a requirement of subcabling between both ends of the flyers and the input of the RDATC's.
Since each flyer can be relatively long, say 200 feet, the need for over 200-400 feet of subcabling per channel or 900 feet or more of subcabling per RDATC defeats one of the major field advantages of the use of intermediate RDATC's within a field system: simplification of the field set-up which links all RDATC's and the central recording truck. Also the use of many different geophone flyers connected by subcabling can lead to foul-ups revolving around the miselectrical connection of the RDATC's and the flyers by the field crew. E.g., the use of large numbers of subcables can lead to some flyers being connected in loops to each other and not to a RDATC. While some helper mix-ups are detectable at the central recording truck some are not; the ones that are, still require communication back from the truck to the "jug handlers" for correction prior to actual recording the seismic data. Hence, loss in crew efficiency occurs. Still yet, each RDATC includes amplifiers whose response can be different dependent on input characteristics. Hence systematic handling of each input channel of data would be desirable.